The invention concerns a hydraulic fluid, in particular for feeding hydrostatic means of support for rolls used for roll-forming aluminum and aluminum alloys.
Rolls with hydrostatic means of support in the interior of the rotating roll shells are such that the hydraulic fluid has to be fed to the said means of support. The inflowing film of hydrostatic fluid between the means of support and the inner face of the roll shells transmits the rolling force and conducts away the heat produced in the deformation process.
The tribo-physical properties required of a hydraulic fluid can be produced by using commercially available products having the characteristics prescribed by the manufacturer of the rolling mill. When rolling aluminum down to foil, however, in spite of all precautions it must be expected that some hydraulic fluid will enter the foil rolling lubricant. Even a relatively small degree of contamination of the rolling lubricant with hydraulic fluid markedly lowers the surface quality of the resultant aluminum foil. Leakage, i.e. of hydraulic fluid into the rolling lubricant, can on subsequent degrease annealing, lead to the following problems with coiled and re-coiled foil: stickiness of the foil strip, non-uniform degreasing of the foil surface, dark colored areas after annealing, white colored spots due to corrosion and contaminated foil surfaces. Residual, thermolytically altered traces of hydraulic fluid left on the surface of the aluminum foil after degreasing in an oxygen containing atmosphere can be detrimental to the properties of the aluminum foil. For example, if such contaminated aluminum foils are employed for impregnated physical capacitors, both the loss angle (tan .delta.) of the spool and the aging behavior are negatively affected. Traces of hydraulic fluid also have detrimental effects on aluminum foils which are chemically or electrochemically etched after annealing and are used for making electrolytic capacitors.
In view of the above it is an object of the invention to develop a hydraulic fluid, which is suitable for feeding to hydrostatic means of support for the rolls and which, when used in roll-forming aluminum, does not lead to any undesirable changes in the aluminum surface, if it enters the rolling lubricant.